This invention relates generally to synthetic array mapping processors and more particularly to the method and apparatus for providing focused line-by-line imagery from coherent sidelooking radar data.
In synthetic array mapping, azimuth resolution is enhanced by periodically sampling the radar data from each range resolution element within the illuminating beam, as the radar antenna is moved along a flight path; and by correlating and integrating (electronic focusing) the received sampled data. This electronic focusing simulates the physical focus (narrow azimuth beam width) of an antenna which approaches the length of the flight path over which the synthetic array was formed, i.e. the path length along with the samples forming the array were taken.
In airborne focused synthetic array radar when mapping broadside along a straight-line flight path, phase corrections have to be applied to the bipolar coherent radar data to achieve signal vector focus. For the received radar data involved in forming a particular synthetic array, the phase corrections are those required to compensate for the two-way quadratically varying range to a particular ground point scatterer. This range variation may be visualized by recalling that the locus of constant range to a given ground scatter is defined by a circle, while the flight path approximates a straight line.
In one type of prior art line-by-line focused synthetic array processor, overlapping successive synthetic arrays are formed by correlating the coherent raw data over an entire array interval at least every time the aircraft has flown a distance corresponding to an aximuth (along-track) resolution element. Each time this correlation takes place, most of the "raw" data from the previous array is resued; the oldest data is dropped out of the correlation process and a new segment of raw data is added. Considering a given range sweep or a particular raw data sample of bipolar echo data, it occupies progressively more rearward positions in each synthetic array as the mapping process continues. For this reason, the amount of phase correction required for a particular raw data sample is not constant, but varies in a quadratic fashion as the particular data element assumes various positions further aft in subsequent arrays. Thus in continuous line-by-line processing dynamically changing phase corrections have been typically applied in the readout of the raw data, not as the data is entered into storage.
Another electronic processing technique which has been employed for implementing a continuous real-time synthetic array "strip map" involves presumming several unfocused groups of returns or subarrays with no phase corrections applied within the subarrays. These unfocused subarrays are then combined to form a total focused synthetic array by phase correcting each unfocused subarray presum as their outputs are added vectorially to form the total array. Due to the "out and back" nature of radar, the magnitude of the "group" phase corrections which must be applied to the particular subarray presums as they are combined to form the total synthetic array, is twice the horizontal (one-way) displacement of the subarrays from the flight path to the circle of constant range. "Group" phase corrections are equivalent to laterally displacing the data perpendicular to the flight path. In the above just described system the phase corrections are applied only to the outputs of the running presums as these presums are added together to form a total array. Thus each presum by itself, before phase correction, consists of undisplaced data gathered along the straight-line flight path. A presum can therefore be continuously updated by dropping off its oldest data as new data is added, then by phase correcting each presum and continuously adding them together, continuous line-by-line imagery is obtained. In regard to this just described technique, when no focusing is done within a subarray, in order for the phase error of all subarrays to be kept reasonable, the subarray lengths must be made shorter in those subarrays which lie outward toward the array extremities. This is because the curvature of the semicircular line corresponding to constant phase (range to a particular ground scatter) is greater at the array extremities. This makes for inefficient processing by requiring a large number of subarrays when quite high resolution (a long total array) is involved.
Another approach to focused line-by-line electronic correlation has involved a sweep integrator such as a recirculating delay line preceded by a programmable phase shifter to accomplish the signal vector rotations. When operating so as to map broadside, this phase shifter has applied to it a quadratic phase (signal vector rotation) program so as to compensate for the two-way range variation between the straight-line flight path and the cord of constant range to a given ground resolution element. When a total array length is completed, the integrator's output sum represents a line of correlated imagery and a particular phase shifter-integrator channel can only then have its data "dumped" and the phase shifter reprogrammed to start the next contiguous array. Thus with only one such channel, only one line of imagery would result per array length. For the just described arrangement to yield successive and contiguous lines of imagery, parallel channels must be provided; each with a quadratic phase program sequence "staggered" with respect to its neighboring channel. With such a multiple channel arrangement, at any instant the channels are in different stages of array completion. Because there must be at least one channel for every azimuth resolution element within an array, very high resolutions dictate a large number of channels; and with many parallel channels the total arithmetic operations involved to accomplish the requisite vector rotations become quite large, particularly at long mapping ranges where the array lengths become quite long.
Another approach to focused synthetic real-time processing has been the so-called "batch" correlation process. In "batch" processing systems bipolar coherent radar returns are analog-to-digital converted, and then applied to a digital memory or store until it is filled. The storage process is then stopped, whereupon the stored data is read out and operated on (correlated) in multiple channels to produce a poly-angle (multiple-beam) two-dimensional imagery "patch". Correlated output imagery produced by "batch" processing is a built-up mosaic of carefully registered small keystone shaped "patches", where each patch is the result of poly-angle correlating a particular "batch" of data. Taken together these registered sub-images comprise a complete image or map. "Batch" processing methods are satisfactory in many applications, but because of the non-rectangular shape of the patches or subframes in certain applications having high resolution and wide swath width (range dimension) requirements, image alignment problems introduce complexities in recording and/or displaying the complete map.